1. Field of the Invention
The present invention relates to an actuator, which is capable of reciprocally moving a slider linearly along a frame under a driving action of a driving source.
2. Description of the Related Art
A transport mechanism such as an actuator has been hitherto used, for example, in order to transport a workpiece.
As shown in FIG. 12, the actuator comprises an inner block 2, which is disposed inside an outer rail 1, and which is displaceable in the axial direction. A ball screw 3 is screw-engaged with a substantially central portion of the inner block 2 in the axial direction.
The outer rail 1 comprises a pair of inner wall surfaces 1a, 1b formed so that the pair of inner wall surfaces 1a, 1b are opposed to the inner block 2. A pair of ball-rolling grooves 4a, 4b, which extend in the axial direction, are formed on the inner wall surfaces 1a, 1b. Unillustrated ball grooves are formed on both side surfaces of the inner block 2 opposed to the ball-rolling grooves 4a, 4b. Return passages 7, within which a plurality of balls 6 are circulated, are formed in the inner block 2. The inner block 2 is guided to make displacement along the outer rail 1 by the balls 6, which are allowed to circulate through the return passages 7, the ball-rolling grooves 4a, 4b, and the ball grooves (not shown).
The ball screw 3, which is integrally connected to a driving source such as an unillustrated electric motor, is rotated, and the inner block 2, which is screw-engaged with the ball screw 3, is displaced linearly in the axial direction of the outer rail 1 under rotary action of the ball screw 3 (see, for example, Japanese Laid-Open Patent Publication No. 2003-074551).
The actuator according to Japanese Laid-Open Patent Publication No. 2003-074551 includes a structure in which a plurality of balls 6 are circulated through the return passages 7 in the inner block 2, the ball-rolling grooves 4a, 4b, and the unillustrated ball grooves, when the inner block 2 is displaced along the outer rail 1 under a driving action of the driving source. However, sliding resistance is generated when the balls 6 are circulated through the ball-rolling grooves 4a, 4b and the ball grooves, and thus abrasion occurs on the inner block 2 and the inner wall surfaces 1a, 1b of the outer rail 1.
Therefore, the outer rail 1 formed with the ball-rolling grooves 4a, 4b and the inner block 2 formed with the ball grooves are each formed of a metal material (for example, stainless steel) capable of being subjected to a heat treatment (i.e., hardening treatment), and both the outer rail 1 and the inner block 2 are subjected to such a hardening treatment. Accordingly, abrasion that arises due to sliding action of the balls 6 is suppressed by increasing the hardness of the ball-rolling grooves 4a, 4b and the ball grooves. However, when the hardening treatment is applied to the outer rail 1 and the inner block 2, additional costs are incurred as a result of the heat treatment, and the number of production steps is increased.
On the other hand, in recent years, demands have grown for lightweight actuators and efforts have been made to further reduce the weight of actuators.